. The railroad engineer's practice . e needed, however, to cer-tainly establish this. In the Howe and Pratt bridges, eachpiece is only strained in one direction, either in tension orcompression. In the Warren truss, however, the same jiiecemay be strained sometimes in one way and sometimes inthe other. The experiments of WOhler tend to confirm thecorrectness of an American practice, that in this case sucha piece should be proportioned to resist the maximumamount of compression ; and should have, in addition,sufficient material to resist the maximum tension. Thenpparent advantage which the Warr
. The railroad engineer's practice . e needed, however, to cer-tainly establish this. In the Howe and Pratt bridges, eachpiece is only strained in one direction, either in tension orcompression. In the Warren truss, however, the same jiiecemay be strained sometimes in one way and sometimes inthe other. The experiments of WOhler tend to confirm thecorrectness of an American practice, that in this case sucha piece should be proportioned to resist the maximumamount of compression ; and should have, in addition,sufficient material to resist the maximum tension. Thenpparent advantage which the Warren has of requiring lesspieces than the Pratt, and making one piece do doubleduty, is thus seen to be only apparent and not real, sincethe braces must contain an additional amount of will perhaps expilain why the Warren truss has neverbeen so popular in the United States as it has bc^n abroad. 6G In a Howe truss bridge the lower chord sticks have tobe spliced, and the very common practice is to makethe splice too This is, perhaps, the commonest form of such splice. The notch A B is generally too small, and bridges areoften seen in which it is crushed. The proper relationbetween the parts of this splice is the followino- : Let c == the crushing resistance of the wood = 5,000 lbs, per square inch for pine. i = the tensile resistance of the wood = 10,000 lbs, per square inch for pine. s = the shearing resistance of the wood = 600 lbs, per square inch for oak and 400 lbs, for pine. Then For pine, we see that A B will equal B ^and just two-thirds of the chord will have to be cut away. The splice-piece and keys are generally made of oak, or other hardwood. This is objectionable, since wood rots much morerapidly when it is in with a different species. Amuch better splice for the lower chord is that recommendedby the late Mr. B. II. Latrobe, in the Railroad Gazette,vol. 10, p, 501. Aflat iron link is set into the chord oneach side. As a veiy small breadth
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